1. Field of the Invention
The present invention relates to a power supply circuit for a portable radiotelephone and, more particularly, to a circuit and method for controlling the power used by a portable radiotelephone.
2. Description of the Related Art
In the field of mobile radio communications, it is possible to interlink two independent cellular radio systems having different operating bands in a method referred to as "dual band method." For instance, in the Republic of Korea (South Korea), it is necessary to interlink the existing CDMA (Code Division Multiple Access) method with the new PCS (Personal Communication System) method, in the U.S.A., to interlink the existing AMPS (Advanced Mobile Phone Service) method with the PCS method, and in Europe, to interlink the existing GSM (Groupe Speciale Mobile) method with the DCS (Digital Communication System) 1800 method.
A cellular mobile radio system which employs the CDMA method is a "digital system. " On the other hand, a cellular mobile radio system which employs a frequency modulation method (FM), such as AMPS, is an "analog system," i.e., speech signals are not digitally encoded prior to transmission on a radio frequency (RF) carrier. In practice, a digital cellular system may be used in dual mode together with an analog system, e.g., a dual mode cellular telephone system capable of being used in both the CDMA mode and the AMPS mode.
A portable (e.g., hand-held) radiotelephone (e.g., cellular phone) which is designed to operate in dual modes generally requires different radiating power levels for each of its modes. As such, each mode requires different battery power to operate. Conventional dual mode portable radiotelephones, however, are designed having power supplies (batteries) that generate power at levels to accommodate the mode requiring the most power to operate. As a result, unnecessary power is dissipated while the portable radiotelephone is operating in the mode which requires less power, which results in a shortened battery life.
Referring to FIG. 1, a diagram illustrating a power supply circuit of a conventional portable radiotelephone is shown. The power supply circuit of the conventional portable radiotelephone includes a power supply 10 ("battery"), a DC-to-DC Converter 20 connected to the output of battery 10, an inductor L1 connected to the output of the DC-to-DC converter, a pair of voltage dividing resistors R1 and R2, which are serially connected between the output of the DC-to-DC converter 20 and a ground, a power amplifier 30 and a linear regulator 31, which are connected to the output of the DC-to-DC converter 20, and a capacitor C1 connected between an input to the power amplifier 30 and ground.
The battery 10 is usually designed to supply more power than that needed by the power amplifier 30 and that needed to operate the ICs (not shown) of the conventional portable radiotelephone. Further, the power amplifier 30 and the ICs generally require different operating voltages. For example, the output voltage of the battery 10 is generally set at 7.2 V, whereas the voltage needed for the power amplifier 30 is 5 V, and the voltage needed for the ICs is 3.3 V. When 7.2 V is applied to DC-to-DC converter 20 from the battery 10, the DC-to-DC converter 20 converts the 7.2 V supplied by the battery 10 to a voltage of 5 V. Such voltage is then applied to the power amplifier 30 to amplify incoming signals and then transmit them through an antenna (not shown). The linear regulator 31 then changes the 5 V supplied from the DC-to-DC converter 20 to 3.3 V, which voltage is applied to operate the ICs.
The output voltage of the DC-to-DC converter 20 is determined by the voltage dividing resistors R1 and R2. In the conventional method, the resistance values of resistors R1 and R2 are preset so that the required 5 V is applied to the power amplifier 30.
In a dual mode portable radiotelephone employing both an FM mode and a CDMA mode, for example, the power amplifier 30 requires 4.7 V when operating in an FM mode, as opposed to 5 V when operating in a CDMA mode, which requires the linearity and saturation characteristics unlike the FM mode. Therefore, by utilizing the conventional power supply circuit of FIG. 1 in the dual mode portable radiotelephone, the 5 V required to power the power amplifier 30 in conformity with the CDMA mode is obtained, as demonstrated above, by presetting the resistance values of the voltage dividing resistors R1 and R2 so as to generate 5 V from the output of the DC-to-DC converter 20. However, because the resistance values remain constant regardless of the operating mode, 5 V is also generated in the FM mode (which only requires 4.7 V). Therefore, when operating in the FM mode, the conventional FM/CDMA dual mode portable radiotelephone unnecessarily wastes power, which results in shortening the battery life.
In addition to the different power requirements of dual mode portable radiotelephones, portable radiotelephones may also transmit signals at different power levels depending on the area in which they are operated. Specifically, the dual mode portable radiotelephone can effectively communicate with low power in a region exhibiting a strong electric field, i.e., in the proximity of a base station, but requires high power to communicate in a region exhibiting a weak electric field, i.e., in a remote place from the base station. In any event, dual mode portable radiotelephones are generally used in close proximity to the base station, i.e, a region of strong electric field. Therefore, on average, dual mode portable radiotelephones can operate with lower transmitting power levels.
In the conventional portable radiotelephone, however, the voltage applied to the power amplifier 30 for amplifying the transmission signals is set at 5 V in consideration of the situation where the portable radiotelephone is operating in a region of a weak electric, which requires higher transmission power levels to communicate effectively. Therefore, since the 5 V applied to the power amplifier remains constant even when the portable radiotelephone is operating in a region exhibiting a strong electric field (which requires less power), unnecessary power is dissipated resulting in shortening the battery life. Moreover, the temperature of the portable radiotelephone increases, which adversely affects the circuitry.